Switching power supplies are used in many electronic systems of varying applications, including telecommunications, networking, test and measurement, and imaging. These power supplies can convert unregulated input into direct current (DC) power at predetermined voltage and power levels and provide a load with either a positive or negative voltage. Switching power supplies typically provide power through low loss components such as capacitors, inductors, and transformers, and by the use of transistors operating in either the cutoff state, where there is a high voltage across the pass unit but no current flow, or the saturation state, where there is a high current through the pass unit but at a very small voltage drop. The transistors essentially create an alternating current (AC) voltage from an input DC voltage. This AC voltage can then be stepped-up or down by transformers and then filtered back to DC voltage at the output.
Early high voltage switching power supplies required the user to manually switch output polarity. To do so, the user had to reconfigure the high voltage circuits by physically disconnecting and connecting key electrical nodes. This practice was unsafe in that it required the user to be exposed to energized high voltage circuits. Another disadvantage to using these power supplies was the significant time required for the user to manually change the output polarity.
Improved high voltage switching power supplies utilized mechanical means to change polarity instead of the manual switching procedure. As an example, new circuitry included control circuits, driver circuits, low voltage relays, high voltage relays, and transformers. Relay control logic was used to control the switching of the relays to produce either a negative or positive polarity. While these more advanced power supplies eliminated the danger of manual switching, the cost of the power supplies increased due to expensive relays. Additionally, even with mechanical switching means, the rate of output polarity change still did not occur at a very high rate. To illustrate, a circuit using high voltage relays generally requires tens to hundreds of milliseconds to switch. Thus, it would take a significant amount of time to warm-up devices that rely upon rapid output polarity change to heat a tube or a lamp used as a heating apparatus. There is also a limited voltage output range available as a result of the constraint in switching speed. For this reason, switching power supplies are commonly used for fixed output or limited adjustment range output designs. It is important for a power supply to be programmable for a wide range of output currents to satisfy the complex and varying needs of today's highly-sophisticated technological devices. A power supply using mechanical switching means does not meet this need.
An additional shortcoming of using mechanical switching means is in the production of bouncing and other undesired effects. A circuit that contains undesired effects is inefficient and can wear out its components after many cycles. This occurrence not only can lead to increased power supply costs for the consumer, but can also lead to the damage of any devices that are connected to the power supply.
A need therefore exists for a for a low power, high voltage power supply that is programmable for a wide range of output current, has the capability of either positive or negative current polarity output, does not use mechanical means for switching polarity, and can be commanded to systematically, efficiently, and effectively change polarity within a few milliseconds without undesired effects that can damage circuit components. Among others, devices that employ a lamp or tube as a heating source will greatly benefit from this type of power supply, as the warm-up time of the lamp or tube will essentially be eliminated.